The invention relates in general to measurements intended to determine at least one characteristic of oil well effluents made up of multiphase fluids, typically comprising three phases: two liquid phases—crude oil and water—and one gas phase. In particular the invention relates to such measurements performed using a composition meter associated or not with a Venturi based flow meter.
The ability of the oil industry to optimise production of a reservoir relies on the possibility of evaluating the well effluent at regular intervals, in terms of quantity (flow rate) and of composition (the proportions of the various phases). This makes it possible to determine what corrective action may need to be taken. However, measuring the flow rate of oil well effluent is a problem that is complex because of the way effluents are usually made up of three phases, and because of the changes in flow conditions to which they are subject (flow rates, fluid fractions, pressure, upstream pipe geometry). These factors give rise to a wide variety of flow regimes being observed, including some regimes of highly non-uniform and unstable character, with the proportions of the phases in the fluid mixture being capable of varying very considerably both in the flow direction (i.e. over time) and across the flow direction, in the form of phase stratification across the flow section.
Numerous proposals based on a Venturi type flow meter have been made to evaluate the well effluent. Amongst those proposals, the international patent application WO99/10712 provides for a Venturi and a gamma ray density meter placed at the throat of the Venturi. The effluent is passed through the Venturi in which it is subjected to a pressure drop. A mean value of the pressure drop is determined over a period of time using pressure sensors and a mean value is determined for the density of the fluid mixture at the throat of the Venturi using the gamma ray density meter. The mean values are used to deduce a total mass flow rate value. Finally further measurements and calculations allow to obtain oil, water and gas flow rates.
While the proposed Venturi flow meter offers a reliable performance in most encountered environments, it becomes difficult to obtain good results at relatively low flow rates. Indeed at low flow rates, the pressure drop measured between the inlet and the throat of the Venturi is becoming too small to provide the flow rate expected accuracy.
Low flow rates may occur when a well produces fewer amounts than expected. This may occur right at the start of measurements or in the course of life of the well.
One solution to the problem of a decreasing pressure drop would be to replace the Venturi flow meter with another Venturi flow meter having a smaller throat diameter. Hence the pressure drop would increase and measurements become more accurate. However the replacement of the Venturi requires to disassemble parts of the Venturi including the gamma ray source, the photomultiplier used to measure the gamma rays and the pressure lines at the inlet and outlet of the Venturi. This makes the replacement a hazardous, expensive and time consuming operation.